Figure 9

Learning as a function of programmed and experienced delays to reinforcement. The imposition of response-reinforcer delays systematically retarded the acquisition of free-operant instrumental responding, but this effect was lessened in H-lesioned rats, even allowing for differences in experienced response-collection delays. (a) The rate of lever-pressing in session 6 is plotted against the programmed response-reinforcer delay. There was a lesion × delay interaction (### p = .001): H-lesioned rats responded less than shams at zero delay (* p = .015), were not significantly different from shams at 10 s delay (p = .197), and responded more than shams at 20 s (* p = .028). (b) Responding in session 6 plotted against the experienced response-to-reinforcer collection delays for sessions 1–6 (vertical error bars: SEM of the square-root-transformed number of responses in session 6; horizontal error bars: SEM of the experienced response-collection delay, calculated up to and including that session). The gradients of the two lines differed significantly (## p = .002; see text), indicating that the relationship between experienced delays and responding was altered in H-lesioned rats.